Use metadata annotation @patch for patch classes

runtime/lib/stopwatch_patch.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // A VM patch of the stopwatch part of dart:core.
 
-patch class Stopwatch {
-  /* patch */ static void _initTicker() {
+@patch class Stopwatch {
+  /* @patch */ static void _initTicker() {
     if (_frequency == null) {
       _frequency = _computeFrequency();
     }
   }
 
   // Returns the current clock tick.
-  /* patch */ static int _now() native "Stopwatch_now";
+  /* @patch */ static int _now() native "Stopwatch_now";
 
   // Returns the frequency of clock ticks in Hz.
   static int _computeFrequency() native "Stopwatch_frequency";
 }